“Airlines are struggling with a lot of complex, legacy technology and it's incredibly challenging to maintain the operational excellence that is expected of them," Dave O'Flanagan, CEO and founder of Boxever, an AI powered software platform for airlines, told NBC News.

"It's not that airlines don't want to improve their technology stacks and modernize to prevent these types of issues. It's just an incredibly complex and expensive task," he said. "They are trying to improve incrementally while still maintaining their ongoing service. To give a flying analogy - it's like trying to change the wing on a plane while the plane is in flight.”

Alex Cruz, British Airways' chief executive, said the surge was so strong that it knocked out the airlines' systems across 170 airports in 70 countries.

"These systems are highly inter-dependent and normally transmit tens of millions of messages a day between different parts of the airline," he said in an update on Monday.

Matthew Gardiner, cybersecurity strategist at security company Mimecast, told NBC News the incident shows that organizations need to pay more attention to the resilience of their systems.

"While localized failures can and do occur, for IT systems to be truly available to meet the operational needs of the organization, no one system failure or localized interruption should be enough to push the system to collapse," Gardiner said in an email. "A global organization such as British Airlines should have a global IT system which matches or exceeds the safety and reliability of its aircraft operations.”

Cruz pledged an "exhaustive investigation into what caused this incident" and said the company would "take measures to ensure it never happens again."

"At the moment, we do not have a complete picture of what happened," Cruz admitted. "Our focus has been on putting things right for the customers affected. When that process is complete, we will hold an exhaustive investigation into the causes of this incident — and do whatever is necessary to ensure it cannot recur."

The airline said on Tuesday that its IT systems were once again operational and a full flight schedule from London's Heathrow and Gatwick airports was planned.

"We are extremely sorry for the frustration and inconvenience customers experienced over the Bank Holiday weekend and thank them for their patience and understanding," the statement said.

Cruz said in a social media update that by the close of Monday, more than two-thirds of the 75,000 customers affected by the delays should have reached their final destinations.

He urged customers to check British Airways' website for the latest information and to learn more about compensation to which they may be entitled. Customers can also learn how to report and track delayed bags on the airline's website.

The fiasco came during a Bank Holiday weekend in the UK — and played out very publicly over social media.

Hi @British_Airways , I had to just book another flight to get home for $3565 because you all canceled my flight home tomorrow ....any help?

The debacle is expected to cost British Airways well upwards of $100 million in compensation payouts, additional staffing, and lost business. Shares in International Airline Group, parent company of British Airways, fell by as much as 4 percent.